The Pope, leader of the world’s 1.2 billion Roman Catholics, has been outspoken on environmental issues. “Climate change is a global problem with grave implications: environmental, social, economic [and] political,” he said in his 2015 encyclical. He also told the UN in the same year: “The ecological crisis, and the large-scale destruction of biodiversity, can threaten the very existence of the human species.”

Butler, from Long Beach, California, has been vegan since the age of six and has staged many protests against animal cruelty. She said there were many reasons for eating a plant-based diet.

The 12-year-old has staged protests against animal cruelty. Photograph: Handout

“There is an animal side, a health side and an environmental side,” she told the Guardian. “If I tell people about the damage that the animal agriculture industry is doing to the planet, then some of them want to go vegan. I also tell them about health and how when you go vegan it can help prevent cancer, heart, disease and diabetes, or how animals have to die or suffer.”

The campaign is backed by celebrities including Paul McCartney, the actor Mena Suvari, the musician Moby and naturalist and TV presenter Chris Packham. Young environmental protesters are becoming increasingly high profile, with Greta Thunberg’s school strikes for climate inspiring tens of thousands of other students to follow suit.

In her message to Pope Francis, Butler says: “In your encyclical letter, Laudato si’, you stated that every effort to protect and improve our world will involve changes in lifestyle, production, and consumption. I agree with all my heart and seek your support in tackling one of the largest underlying causes of the problems we face: animal agriculture.”

Matthew Glover, CEO of Million Dollar Vegan and who co-founded the Veganuary campaign, said the Pope had been selected for his environmental leadership and influence, particularly in South America, where great forests continue to be cut down.

“We are launching this deliberately bold, audacious campaign to jolt our world leaders from their complacency,” Glover said. “We are thankful that Pope Francis has spoken out and that is why we are humbly asking him to try vegan for Lent, and set an example of how each of us can align our principles of caring and compassion with our actions.”

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The $1m has been donated by the Blue Horizon International Foundation, the charitable arm of the Blue Horizon Corporation, which invests in companies producing plant-based foods and aims to “accelerate the removal of animals from the global food chain”.

Lent is a period of about 40 days that ends just before Easter and during which many Christians practise abstinence of some kind. It reflects the Bible story of the 40 days Jesus spent fasting in the desert and in 2019 runs from 6 March to 18 April. The Vatican had not responded to a request for comment at the time of publication.

Daniel Hale, at the Catholic aid agency Cafod, said: “Pope Francis has been the standout world leader on climate change this decade. Like all Christians, the Holy Father will make his own choices about how to give, act and pray this Lent.

“It’s a period of reflection for all Catholics and a chance for us to respond to the Pope’s call for us to live a simpler and more sustainable lifestyle in various ways,” he said. “For some, that will mean cutting down on meat, while others are switching to renewable energy or campaigning for action to reduce emissions.”

Packham said: “We’ve made an enormous mess of this precious little planet in all sorts of ways. [Million Dollar Vegan] is important, brilliant and audacious. If the head of the Catholic church can be vegan for [Lent], think of the message that will send to all of those followers across the world.”

Butler’s activism began early. “When I was about to turn four I asked my mom about where we got our chicken nuggets from, because I would always eat them,” she said. “I was devastated. I told her that I never wanted to eat meat again. But I still love chicken nuggets – the vegan version – and I love vegan mac’n’cheese.”